The present invention relates to determining whether a plurality of objects have been properly placed in a holding device, and particularly to such apparatus which optically inspects placement of the objects.
During semiconductor fabrication, wafers of silicon are held in a cassette between processing steps. The cassette typically is a frame with a plurality of horizontal slots, each of which receives a silicon wafer. A handling mechanism within the fabrication system individually removes each wafer from the cassette for processing and returns it to the cassette after processing.
Because of the automated handling of the wafers, it is necessary to inspect the cassette to insure that all of the wafers are properly placed into the slots of the cassette. Due to placement inaccuracies or non-uniformity among the wafers, such as chipped and flat edges, the wafer may fall out of its slot and fall onto the wafer immediately below in the cassette. Alternatively, one side of the wafer may be misplaced or drop out of its slot while the opposite side remains in the slot so that the wafer is slanted in the cassette. Improperly located wafers can be damaged by the wafer handling device and result in damage to other wafers.
Previous semiconductor wafer processing equipment had optical systems for detecting wafers which were misaligned within the cassette. Typically, the optical systems had a light source on one side of the cassette to transmit the light beam through the cassette and had a detector on the opposite side that responded to the transmission of light. The wafers blocked the light beam, enabling the presence of a wafer to be detected. Some systems had several of these emitter-detector pairs which sent a plurality of light beams through the cassette. Such detection systems were difficult to incorporate into the semiconductor processing equipment since they required individual windows on each side of the equipment so that the light could pass therethrough. In addition, wires had to be run around the processing equipment between the light source and detector which often may be difficult depending upon the physical installation of the equipment.